Clinical decisions regarding patient management are often made on the basis of blood chemistry analysis. A variety of procedures have been used to perform such analyses, all of which have their deficiencies.
Blood chemistry is often determined on a drawn sample of blood which is subsequently transported to an on-site facility where the analysis is performed. Blood chemistry analysis performed by such a process engenders a risk of contact with the blood sample, an increased risk to the patient of nosocomial infections and the possibility that air emboli may be introduced into the bloodstream, a potential for contamination of the sample, and, perhaps most significantly from the diagnostician's point of view, a lengthy delay between a decision that blood chemistry is necessary and delivery of therapy based on the results of the analysis.
The need for a bedside system to analyze critical blood variables (e.g., O.sub.2, CO.sub.2 and pH) has been addressed by placing environment-sensitive, calibrated optical or electrochemical sensors directly into a patient's artery or vein. Intraarterial or intravenous sensors are typically calibrated by immersion in a solution which has been equilibrated by bubbling with known concentrations of, for example, O.sub.2 and CO.sub.2, to provide a liquid with known partial pressures of O.sub.2 and CO.sub.2 (i.e., pO.sub.2 and pCO.sub.2). The ability of the sensors to detect pO.sub.2 and pCO.sub.2 is then compared with the known pO.sub.2 and pCO.sub.2 ; this process is referred to as calibration by tonometry.
A major disadvantage of this system is that once a calibrated sensor is inserted into a patient's blood vessel, it must be removed from the vessel for re-calibration and sterilized again before it can be re-inserted. Furthermore, it is equally difficult to perform quality control measurements to determine whether the sensors are functioning properly. Absent the ability to re-calibrate, it is extremely difficult to determine whether the system is performing properly after the sensors have been inserted. In fact, poor performance is frequently seen since (1) intraarterial or intravenous sensors are prone to thrombogenic formations which can cause serious measurement errors and (2) patient movement can result in sensor contact with the vessel wall which can also cause temporary or permanent measurement errors.
An alternative approach is a paracorporeal or extracorporeal system for bedside blood chemistry analysis. The paracorporeal system places the sensors in a physiologic line very near to a patient's arterial catheter. This approach has the primary advantages of eliminating the problems associated with thrombosis and patient movement and, in addition, provides the capability to conduct in situ calibration and quality control checks without compromising sterility.
A paracorporeal design allows for a calibration to be made while the sensors are either in the physiologic line (e.g., arterial or venous line) or removed from the physiologic line (i.e., ex vivo). Moreover, quality control checks may be made at any time throughout the life of the sensors.
Accordingly, novel methods of conducting in situ or ex vivo calibration and quality control testing are provided for use with sensors positioned either in a physiologic line or separate from a physiologic line.